Pharmacokinetic interactions among phenobarbital, carbamazepine, and valproic acid in pediatric Japanese patients: clinical considerations on steady-state serum concentration-dose ratios.

: With antiepileptic drugs, the marked inter- and intrapatient variability of the concentration-dose ratio makes it difficult to predict serum concentrations from the dose administered per kilogram. The effects of comedication on steady-state serum concentration/dose ratios of antiepileptic drugs were evaluated retrospectively in 669 pediatric patients. To avoid complex pharmacokinetic interactions among multiple antiepileptic drugs, the data on serum concentrations in the current study were collected from patients who were co-administered only one additional antiepileptic drug (phenobarbital-carbamazepine, phenobarbital-valproic acid, or carbamazepine-valproic acid) or who received monotherapy. The concentration/dose ratio for the antiepileptic drugs increased significantly with body weight in children up to 15 years old. Associated therapy with antiepileptic drugs affected the concentration/dose ratio. Therefore, routine monitoring of serum concentrations of the antiepileptic drugs is extremely useful, particularly in children and in patients who require associated antiepileptic medication.